Moraxella catarrhalis is a Gram-negative human mucosal pathogen, a significant cause of middle ear infections and sinusitis in infants and children and an important source of exacerbations in adults with lung disease. It is the third leading cause of otitis media (OM) and there is a high colonization rate with M. catarrhalis, as 50% of children will be colonized in the first 6 months of life. The incidence of OM is also high and as about 80% of all children under under 3 will experience at least one episode. In addition, recurrent acute OM is prevalent resulting in hearing impairment and developmental/learning problems as these children reach school age. OM is a significant source of direct and indirect health care costs and M. catarrhalis is responsible for 3 to 4 million physician office visits annually. This is considered low as diagnosis is difficult because tympanocentesis is uncommon. These facts have stimulated research aimed at identifying virulence factors involved in pathogenesis. One of the prominent surface components implicated as a virulence factor is the lipooligosaccharide (LOS). Structural analysis confirmed that M. catarrhalis LOS shares similarities with the LOS of Gram-negative human pathogens, i.e. Neisseria meningitidis, N. gonorrhoeae and Haemophilus influenzae. More importantly, these common carbohydrate epitopes are involved in virulence, i.e. adherence, biofilm formation and serum resistance. To date there has been little effort to evaluate the role of M. catarrhalis LOS in disease. The most likely reason for the lack of research in this area involves the fact that there was no data describing the genes involved in the biosynthesis and assembly of M. catarrhalis LOS and there were no defined mutants that could be used to delineate the importance of different oligosaccharide structures. We have made significant progress in this area over the previous period identifying 17 LOS genes and constructing each specific mutant. In addition, we have proposed a potential animal colonization model, together with our collaborators, which could be critical in performing in vivo studies with M. catarrhalis. We are now in a position to continue genetic and biologic studies designed to test our hypothesis that the serotype LOS structure assembled by M. catarrhalis isolates is directly related to pathogenesis. We further hypothesize that the serotype A and serotype B LOS structures may provide an advantage over the serotype C LOS structure. We will test our hypotheses by the following specific aims: (1) Determine the sequential steps involved in the assembly and biosynthesis of M. catarrhalis LOS and (2) Determine the role of M. catarrhalis LOS structures in pathogenesis.